CHAPTER IX 

 CLASS VI. MAMMALIA 



Unfortunately the only vernacular name for the class Mammalia 

 is mammals, but the man of the street does not know what a mammal 

 is. He knows birds, reptiles, fishes and has an idea that a frog is an 

 amphibian; but he uses a variety of words to express his idea of a 

 mammal, none of which seems to serve the purpose very well. He 

 sometimes uses the word "beast," but this term does not seem to 

 apply to men, at least not to all men, nor to whales; he uses the word 

 "quadruped," but this term seems scarcely appropriate to bipeds, 

 whales or bats. 



It has generally been assumed that mammals represent the apex 

 of organic evolution, or at least that of the chordate phylum. It is, 

 however, not to be granted as axiomatic that the mammals represent 

 a higher level of evolutionary attainment than do the birds; for the 

 birds are a more recent evolutionary product, are more nearly a 

 climax group to-day, and on the whole represent a more highly spe- 

 cialized condition than do the mammals. In only one particular do 

 the mammals exhibit a distinctly higher order of specialization than 

 do the birds; namely, in brain specialization, and particularly in 

 that of the cerebral hemispheres. It has also been said that the 

 mammals surpass the birds in specialization of the teeth and of the 

 feet. This is true in a sense, though the toothless condition of the 

 bird and the replacement of teeth by the bill is really a more highly 

 specialized condition than any in which the teeth still persist; while 

 the wing represents an extreme specialization of the fore limbs more 

 radical than anything in the mammals, except possibly the flippers 

 of whales. It must be admitted, however, that the bird's hind limbs 

 are rather conservative; for the wing has rendered the functioning of 

 the foot of secondary importance. The claim of the class Mammalia 

 to supremacy in taxonomic ranking rests almost entirely upon their 

 superiority of nervous organization. Man as the exemplar of brain 

 specialization adds immeasurably to the claim for supremacy of the 

 class to which he belongs; for there is no dispute as to the supreme 



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